have you been playing with modsecurity lately? that could explain it - eg. modsecurity was blocking your request because of the short_open_tag, then you reinstall, which disabled modsecurity in the ispconfig vhost, so it works again. If that's the case, there's a post at https://www.howtoforge.com/community/threads/mini-howto-modsecurity-crs-3-0-on-debian-jessie.74898/ where you can take up the issue, where you can (rather, should? post the modsec rule or log entry that you're hitting to be added as an exception. hopefully the work there will make it into the official modsecurity crs3 ruleset in the future.